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In the HTML editor, in an <object> element, if the "code" attribute is used, the editor gives a warning: The "code" attribute on the "object" element is obsolete. Use the "data" and "type" attributes to invoke plug-ins. This advice is correct, according to w3c documents. However, if the "code" attribute is changed to the "data" attribute, when the applet is invoked using the NetBeans appletviewer (right-click "Run"), the only output is: Warning: <object> tag requires code attribute. If the "data" attribute is changed back to "code" then the applet runs.
So the issue is not about HTML validation, but about Applet Viewer. Maybe you are using it wrong? Reassigning to applet component.